While driving on the Capital Beltway at 6:30 one morning this month, Carl Pennybacker glanced up from the side-view mirror of his tanker truck and was greeted with a stomach-wrenching sight: brake lights.

As often happens on the region's most crowded highway, traffic suddenly had slowed without warning. At a speed later estimated at 40 miles an hour, Pennybacker's truck plowed into the back of a Ford pickup carrying three construction workers, crushing it all the way to the cab.

That was just the beginning. The pickup smashed into a Volkswagen Scirocco that in turn smacked the back of a Chevrolet Silverado. The Chevy hit a van, the van hit a Volkswagen Rabbit, the Rabbit hit a Nissan 200SX, the Nissan hit a Datsun 510, the Datsun hit a Chevy pickup.

When the chain reaction finally subsided, three vehicles had been destroyed, two others needed a tow truck, and four persons required ambulances, although no one had been seriously injured.

The traffic jam stretched for miles.

As accidents on the Beltway demonstrate all too often, cars and trucks can be a dangerous -- sometimes deadly -- combination.

In recent months, as traffic congestion on the 66-mile highway has reached new peaks, that volatile mix has become the focus of unprecedented concern.

"There is a problem unique to the mix of passenger cars and trucks," said Hal Kassoff, chief of the Maryland Highway Administration.

"A combination of the density of the traffic, the mix {of traffic} and the expectation that one ought to be able to drive at the speed limit, or past it, just produces a situation where the accident rate starts to grow."

For a variety of reasons -- size, braking ability, blind spots -- tractor-trailers are more accident-prone than other vehicles, safety experts say. Tractor-trailers account for about 3.5 percent of the traffic on the Beltway but are involved in 18 percent of the accidents, according to the Metropolitan Washington Council of Governments.

In concert with a nationwide crackdown, Maryland and Virginia are taking steps to reduce the number of truck wrecks on the Beltway. Among the steps are an extension of a ban on trucks in the left lane of the I-95 portion of the Beltway, more truck inspection stations, beefed-up truck enforcement patrols by state police and longer exit and entrance ramps at key interchanges.

By all accounts, this is just the beginning. A recent consultant's study on the Beltway commissioned by the Virginia Department of Transportation raised the idea of separate, parallel truck roads in congested areas.

Maryland officials have suggested placing heavy-duty wreckers at the Woodrow Wilson Bridge, the Beltway's most notorious choke point. Another plan would use video cameras to monitor the bridge for accidents.'Something Good'

The efforts have started to show results. In the first six months of this year, Maryland recorded 241 accidents involving tractor-trailers or heavy trucks on its portion of the Beltway, down slightly from 249 during the same period last year.

Virginia recorded 99 such accidents during the first half of this year, down from 113. The declines came despite an average annual traffic increase of about 7 percent on some sections of the Beltway.

"When traffic volumes increase as much as they have, you would expect to see a corresponding increase in accidents," said John L. Butner, an assistant traffic engineer with the Virginia Department of Transportation. "So something good is happening."

But the problem is not likely to go away. Despite reams of bad publicity and official outrage, some truckers persist in reckless behavior behind the wheel -- from speeding to leaving the scene of an accident to, on rare occasions, drunken driving, according to accident reports and safety studies.

Police estimate that, at any given time, nearly half the trucks on the Beltway would not pass a safety inspection.

Moreover, the risk of getting caught is slight. State troopers say the Beltway is so crowded they often have to let infractions go unchallenged. In Maryland last month, State Police officials decreed that radar traps not be set on the Beltway during rush hour; it was too disruptive to traffic flow, according to Trooper Lori Harris.

Safety is only part of the concern. When things go awry on the Beltway, thousands of drivers are delayed. "In Maryland and Virginia, the {traffic} volumes have increased so dramatically that any incident creates a ripple effect," Butner said.

Some contend the safety threat is exaggerated. The fatal-accident rate is lower for the Beltway than for the nation's interstates, largely because traffic on the Beltway moves so slowly so much of the time, safety experts say. Perceptions, Reality

To the trucking industry, and to many law officers, the main problem is one of perceptions. "The reality is that, considering the volumes of traffic, the Beltway is one of the safest highways in the nation," said Capt. Herbert Northern of the Virginia State Police. "That's not to say that it's not scary, that people in small cars aren't frightened."

Trucking industry spokesmen point out that the Beltway was built to serve interstate traffic -- not local drivers, who now account for more than two-thirds of its users. They note that trucks play a vital role in the economy.

"It takes eight trucks a day to keep a medium-sized supermarket stocked," said Thomas J. Donohue, president of the American Trucking Association.

But Donohue says the industry is worried that unsafe driving practices are giving it a bad name. His trade group lobbied hard for a new federal law that prohibits truckers from holding multiple driver's licenses, a common ploy that allows drivers to spread violations over more than one state.

"In reported accidents, tractor-trailers are at fault probably two out of three times," he said. "We have too many accidents with trucks."

In a 1985 study of Beltway accidents, the American Automobile Association found that truckers were charged in 68 percent of accidents that also involved cars; ordinary motorists, by comparison, were charged 35 percent of the time. A Congested Stretch

Still, most law officers and transportation officials are sympathetic to the truckers. In their view, the Beltway's biggest problem is traffic congestion, not recklessness. On the stretch between Arlington Boulevard and I-66 in Virginia, traffic increased 70 percent between 1975 and 1985, from 91,500 vehicles per day to 158,400, according to the Virginia Transportation Department consultant's study.

"The thought of myself in a tractor-trailer is totally frightening," said Lt. Michael Barnes, commander of the College Park barracks of the Maryland State Police. "You can visualize the frustration: People pull in front of you, then you've got to stop an 80,000-pound vehicle."

The tanker accident that occurred on the outer loop of the Beltway near Connecticut Avenue this month was, in many ways, a case study.

The driver was guilty of no egregious error. He had not been speeding, drinking or driving recklessly. His truck showed no evidence of faulty equipment. All he had done, according to Trooper Harris, who investigated the crash, was allow his gaze to linger too long in his side-view mirror while making a routine lane change.

"He was checking his blind spots, he looked up from his mirror and WHAM!" she said.

Luckily for those driving in front of the tanker, it was empty. "If he had been fully loaded, I don't think anyone in that pickup or Scirocco would have survived," Harris said. "There would have been skid marks up to the fifth car."

The trucker received a ticket for $40. "He knew he was at fault," Harris said. "He was just glad no one got hurt." Sideswiping Most Common

According to the 1985 AAA study, the most frequent cause of accidents involving trucks and cars was sideswiping. The charge most often placed against truckers, according to the AAA, was failing to drive in a single lane or making an improper lane change.

"I didn't even see that woman at all," said Wayne Black, a truck driver from Townsend, Tenn., describing his encounter with a 1981 Chevrolet Malibu on Oct. 14 near the Baltimore Avenue exit on the Beltway.

"On the front right corner of your truck, you can't see if there's a car right there beside you," Black said. "A lot of times I'll really look over my steering wheel and sort of hesitate before I go over." This time it didn't work. "She was right there," he said.

Black, who was hauling a load of cigarettes, was cited for making an improper lane change.

Maryland State Trooper David Frank has seen so many sideswiping accidents that he has developed a strategy for dealing with irate motorists. "I stick them up in {a truck cab} so they can see what the driver sees," he said. "They can't put enough mirrors on those tractor-trailers to cover every blind spot."

Another common type of accident is the infamous "rear-ender," which accounted for about 20 percent of all truck-related accidents on the Beltway in 1985, according to the AAA. The consequences can be disastrous.

On Sept. 13, Matilda Pollard, an apartment manager from Annandale, was driving to Baltimore with two priests, one of them her brother, on the inner loop of the Beltway when she glanced in her car's mirror and saw a truck approaching.

"It was one of those 18-wheelers," she said. "It was going so fast and I wanted to go left or right but I couldn't because of the cars."

A moment later, the 1979 Peterbilt, driven by Kenny Hubert Watkins of Martinsville, Va., hit the rear of her four-door sedan. The car flipped over and flew through the air; the truck began to overturn and struck three other vehicles. Then, as it skidded off the road, it struck Pollard's sedan a second time.

Watkins was killed; Pollard suffered a concussion. "I think it made a difference being with two priests," she said.

Police later found that the truck's brakes were defective, which could have contributed to the crash. It wouldn't be the first time. According to the AAA, faulty equipment, or loose or overweight loads contribute to about 6 percent of all Beltway truck accidents.

Hit-and-runs are another pattern in truck-related accidents. The 1985 AAA study found that once every 10 days a tractor-trailer struck another vehicle on the Beltway and left the scene.

Lillian Tyree and her husband know what that feels like. Last month, the Mitchellville, Md., couple was driving on the Beltway near Annapolis Road when a tractor-trailer roared alongside.

"He looked like he was going to come into our lane, then realized we were there, but before he could swerve back, he sideswiped us," she said. "It was very frightening."

Tyree said the driver knew he had hit their car. "We saw him face-to-face," she said. "He shrugged his shoulders and kept going."

Frank Burgess, part owner of an ironworks in Capitol Heights, had a similar encounter Sept. 17.

"He kind of smiled," Burgess said of the trucker who pulled up close behind him, then changed lanes to pass and sideswiped him as he went by. "He just came flat over," Burgess said. "He didn't even slow down."

Burgess wrote down the rig's tag number, but authorities told him it had expired and could not be traced. "Tractor-trailers on the Beltway stink," Burgess said. "They do what they want and nothing ever gets done."

Not every trucker is at fault. Accident files are rife with examples of truck-related accidents caused by motorists, such as the owner of a Porsche 944 who cut in front of a tractor-trailer on the Beltway near New Hampshire Avenue on Sept. 11. The Porsche had to be towed away and the owner got the ticket.

Trooper Frank complains that many drivers take advantage of trucks on the Beltway. "In heavy traffic, trucks will leave 15 to 30 feet in front of them" in case traffic comes to a sudden stop, Frank said, and motorists "use it to get in front."

Donald Estes, who hauls refrigerated goods around the country from a depot in Columbus, Ohio, says few people realize how hard it is to stop a truck in an emergency. "Fully loaded, my truck weighs 80,000 pounds," he said. "It's a heck of a lot easier to stop that car than 40 tons."

To Donohue, the trucking association president, few things are as infuriating as the sight of a small car, full of passengers, dodging in and out of truck traffic on the Beltway.

"I was going home the other night and a car with five or six kids in it pulled right in front of a tractor-trailer," he said. "As soon as they did that, traffic began to slow and {the truck driver} pulled into the left lane. But if there had been no place to go and traffic stopped quickly, I want you to read the headline in The Washingon Post: 'Tractor-Trailer Kills Six Kids.' "

Numerous studies have documented the hazards of trucks on urban freeways. According to a 1986 study by the New York City Department of Transportation, "Increasing truck weight and dimensions, coupled with shrinking passenger car size, serve to increase the likelihood of tragedy when cars and trucks collide."

The Virginia Transportation Department consultant's study found that the rate of accidents involving trucks on the Beltway is more than four times that of accidents involving only passenger cars. In addition, 44.2 percent of accidents involving trucks are likely to cause injuries or death, compared with 37.5 percent of Beltway accidents in general, according to the report.

Beyond the safety aspects, commuters pay a huge price in lost time and frustration.

In June, for example, a predawn accident involving three tractor-trailers at the Wilson Bridge tied up traffic throughout the metropolitan area at the height of rush hour, delaying some commuters as much as an hour and a half. A field sobriety test showed the driver who caused the crash had been drinking.

More recently, two tractor-trailers jackknifed on the Maryland side of the Wilson Bridge during the freakish preseason snowstorm that descended on Washington earlier this month. The trucks prevented plows and tow trucks from clearing the bridge of snow and abandoned vehicles, and helped create a tie-up that lasted more than 12 hours.

Transportation officials hope they can solve some of those problems through better management of the highway. Virginia, for example, wants to establish a fleet of private tow trucks that would be on call during a snow emergency to prevent the kind of situation that tied up the Beltway two weeks ago. Another effort at management is the ban on trucks in the left lane of the I-95 portion of the Beltway in Maryland and Virginia.

But bureaucratic solutions have their limits. During the first 24 months the truck restriction was in place, the accident rate for all vehicles on that portion of the Beltway increased slightly, according to the Virginia Transportation Department. Nonetheless, the department decided the restriction should be extended, citing "various intangible benefits such as favorable public perception."

More ambitious solutions such as the construction of separate truck roads are, at best, a long way off and would likely cost in the tens of millions of dollars.

Estes, the truck driver from Columbus, Ohio, cannot wait. "Anything that separates cars and trucks, I'm all for it," he said.